1. Field of the Invention
The present invention relates to a crankcase compression type stratified scavenging two-stroke cycle engine, and more particularly to an air-ahead stratified scavenging two-stroke cycle engine.
2. Description of the Related Art
In crankcase compression type stratified scavenging two-stroke cycle engines, air-fuel mixture produced in a carburetor is introduced into the crank chamber and air is introduced into scavenging passages connecting scavenge ports of the cylinder to the crank chamber utilizing negative pressure produced in the crank chamber by upward movement of the piston toward the top dead center.
On the other hand, an air-fuel mixture in the cylinder is compressed as the piston moves upward toward the top dead center.
When the compressed mixture in the cylinder is ignited neat the top dead center, the combustion gas of the mixture increased in pressure pushes down the piston, and the combustion gas is exhausted through the exhaust port when the exhaust port is opened by the downward moving piston. Pressure in the crank chamber rises as the piston moves down toward the bottom dead center, and first the air in the scavenging passages is pushed into the cylinder through the scavenge ports of the cylinder, then the mixture in the crank chamber is pushed into the cylinder through the scavenging passages following the air pushed into the cylinder to scavenge the combustion gas in the cylinder. Thus, scavenging is first done with air and then followed by the mixture, by which blow by of mixture through the exhaust port is prevented. When the piston moves upward, the scavenge ports and exhaust port are closed by the piston and the mixture in the cylinder is compressed to be ignited near the top dead center. A stratified scavenging two-stroke cycle engine as mentioned above is disclosed in Japanese Laid-Open Patent Application No. 2001-254624 (patent literature 1).
According to the patent literature 1, as shown in FIG. 1 of the patent literature 1, a passage connecting member made of heat insulating material is provided between the cylinder and the carburetor. The carburetor has a venturi and an air passage. Mixture produced at the venturi of the carburetor is sucked by the negative pressure in the crank chamber into the crank chamber through a mixture flow passage in the passage connecting member and a mixture flow passage connecting to a mixture suction port, which port is opened by the piston when the piston moves up from the bottom dead center to be communicated with the crank chamber. Air is sucked by the negative pressure in the crank chamber through the air passage of the carburetor and an air flow passage in the passage connecting member into an air supply camber formed at a side of the cylinder, from where further into scavenging passages which are connecting to the crank chamber at their lower ends and connecting to scavenge ports in the cylinder at their upper ends. A throttle valve is provided in the venturi and an air flow control valve is provided in the air passage of the carburetor. A check valve is provided between the air flow passage in the passage connecting member and the air supply chamber at the cylinder. The check valve allows the air in the air flow passage in the passage connecting member to flow only toward the air supply chamber and prevents a reverse air flow from the air supply chamber to the air flow passage in the passage connecting member.
The check valve is a leaf valve provided to interrupt the air in the scavenging passages from flowing through the air supply chamber toward the air flow passage in the passage connecting member when the piston moves down and the air in the scavenging passages is pushed into the cylinder.
The structure around the check valve of the stratified scavenging two-stroke engine disclosed in the patent literature 1 will be explained referring to FIGS. 4 to 6. FIG. 4 is a sectional view, FIG. 5 is a front view viewed from the cylinder side in FIG. 4, and FIG. 6 is an exploded perspective view.
In the drawings, reference numeral 10 is a check valve set, 1 is a cylinder, 2 is an passage connecting member made of heat insulating material, 3 is a carburetor, 4 is a passage connecting member gasket provided between the cylinder 1 and passage connecting member 2, 5 is a carburetor gasket provided between the passage connecting member 2 and carburetor 3, and 6 is a mixture flow passage and 7 is an air flow passage in the passage connecting member 2. Reference numeral 1a is a flange part formed at a side of the cylinder 1 to which the passage connecting member 2 is attached. Reference numerals 1b and 1c are respectively an air supply chamber and a mixture passage formed in the flange part 1a. Reference numeral 1d is a combustion chamber in the cylinder 1, 1e is a mixture inlet port, 2a is a flat face of the passage connecting member 2 for connecting it to the flange part 1a, 2b is a flat valve seat face protruded from the flat face 2a, 11 is a leaf valve, and 12 is a stopper which is fixed to the passage connecting member 2 together with the leaf valve 11 by a screw 13. As shown in FIGS. 5 and 6, the leaf valve 11 and stopper 12 are fixed to the passage connecting member 2 at two holes provided at the upper part thereof, one of the holes is engaged with a locator pin 2c put in the passage connecting member 2 and the screw 13 fastens the leaf valve 11 and stopper 12 together to the passage connecting member through the other hole.
The leaf valve 11 closes the air flow passage 7 in the insulator 2. When negative pressure is produced in the air supply chamber 1b, the leaf valve 11 bends as shown by chain line in FIG. 4 pulled by pressure difference between the air flow passage 7 and the air supply chamber 1b, and further bends pushed by dynamic pressure of air flow that occurs when the leaf valve bends and the air in the air flow passage 7 flows into the air supply chamber 1b. 
The stopper 12 restricts the bending of the leaf valve 11 to determine a maximum opening of the leaf valve 11.